The present invention is related to Multi-parameter Sensing based on Few-mode Fiber Bragg Gratings using Femtosecond IR Laser.
Fiber Bragg grating (FBG) is a periodic perturbation of the refractive index along the fiber length. Thanks to their unique filtering properties and versatility as in-fiber devices, FBGs have been widely investigated as sensing elements for a variety of applications, including structural health monitoring of dams, highways, bridges, railways, aircraft wings, as well as spacecraft fuel tanks. Especially among the harsh environmental sensing applications of FBGs, high temperature sustainable gratings have been an extremely attractive topic. Nonetheless, due to the weak bonds of germanium and oxygen in the optical fiber medium, the maximum working temperature for conventional FBGs is merely around 650° C., even with the help of metallic coatings. However, numerous applications in the fields of industrial process control, electrical power industry, automobiles, and the defense sector, may create emergency situations at or above 1000° C. Therefore, the long-term thermal stability of the FBGs in a harsh, high temperature environment has become one of the main practical challenges of FBG's real-world applications. Another key issue includes the discrimination of different sensing parameters such as pressure, temperature, bending, displacement, magnetic field, current, rotation, acceleration, vibration or chemical concentration. In particular, the cross sensitivity of temperature and strain is a crucial issue in high-performance optical fiber sensor design, which would introduce additional error when measuring each of them independently, for most of the sensing components are both sensitive to temperature and strain simultaneously. There is therefore a need for new optical fiber sensor systems capable of operation from ambient temperatures up to 1000° C., as well as differentiating various sensing parameters such as temperature, strain and pressure etc.
Various methods have been employed in an attempt to increase the maximum usable temperature of FBG sensors, such as drawing specialist ion-doped fibers or chemical composition fibers. Nonetheless, these approaches are too costly, thus lack of applicability and replicability. Others attempted the formation of the damage written (type II) gratings, which is inscribed by multi-photon excitation with higher intensity lasers. FBG was usually formed by exposure of the core to an intense optical interference pattern, with an intense ultraviolet (UV) source such as a UV laser as the irradiation source. As shown in FIG. 1, FBG can be made using holographic approach with bulk interferometer. The UV beam is divided into two by the beam splitter, and then brought together again by the two UV mirrors. The two UV beams interfere and write a pattern on the photosensitive fiber. The refractive index of the core changes with exposure to UV light, with the amount of change depending on the exposure intensity and duration, since the germanium-doped fiber is photosensitive. However, one of the biggest limitations of such UV-induced gratings is that, due to the diffusion and/or degradation of the defects or fiber impurities, any elevated temperatures above roughly 250° C. may lead to the erasure of the UV-induced index modulation of the grating, thus the range of operation temperature of these sensors was strictly delimited. The fiber gratings inscribed by femtosecond IR laser have annealing characteristics similar to the type II damage written fiber gratings, where the short timescales offer unprecedented spatial localization of the induced change. This new FBG fabrication method is based on refractive index change modification by high intensity femtosecond laser radiation applied to optical fiber, thus ensures stable operation at temperatures as high as 1100° C. Nevertheless, single-mode silica fiber Bragg grating sensor (SM-FBG) inscribed by femtosecond pulses may still be unable to discriminate between different effects, temperature and strain for instance, due to the fact that various physical and chemical changes can impact the FBG wavelength simultaneously.